An eternal skeptic, Sigmar Polke questioned everything—from the artist’s very role to the necessity of paint in a painting. His doubt was productive: When he died, at 69, he left behind a vast body of photographs, films, drawings, prints, sculptures, and paintings. Nearly 300 works will be on view in "Alibis: Sigmar Polke 1963–2010,” one of the most comprehensive looks yet at the pranksterish artist, who came of age in postwar Germany. The exhibition, which runs from April 19 through August 3 at New York’s Museum of Modern Art, will include “paintings” made with soot and photochemicals, as well as never-before-seen films created by the artist under the influence of hallucinogens in the ’70s. Painting while high on LSD, Polke made a mockery of artistic intent as he churned out one ravishing picture after another.